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Migrazioni delle specie ittiche - Il giornale del mare
Skip to content Sono poche le specie ittiche in cui gli individui rimangono in una stessa area per tutto il corso della loro vita (specie sedentarie ). La maggior parte dei pesci effettua migrazioni su scala diversa nel corso del ciclo vitale. Questi possono essere piccoli spostamenti locali e legati ad attività che contribuiscono alla crescita, sopravvivenza e riproduzione, oppure possono essere vaste migrazioni oceaniche o tra ambienti differenti.
Con il termine migrazioni si intende ogni movimento direzionale di massa da un’area a un’altra, che abbia caratteristiche di regolarità nel tempo o in relazione con la fase biologica. La maggior parte delle specie ittiche compie migrazioni in relazione al ciclo riproduttivo (migrazioni genetiche ), all’accrescimento e in alcuni casi all’alimentazione (migrazioni trofiche ). Queste migrazioni sono stagionali e legate al tipo di strategia riproduttiva che prevede la produzione di uova pelagiche.
L’importanza delle aree di nursery Affinché le larve e successivamente i giovanili abbiano possibilità di incontrare risorse sufficienti per sopravvivere è necessario che le aree in cui avviene il loro accrescimento abbiano caratteristiche idonee. La dispersione delle larve è quindi un momento cruciale nell’ambito del ciclo vitale di una specie e non avviene in maniera casuale. Ogni popolazione si riproduce in aree ben delimitate in relazione alla distribuzione delle correnti locali, in modo da assicurare che le larve vengano trasportate in una direzione ben determinata verso le aree marine in cui si concentreranno i giovanili (aree di nursery ).
Migrazioni dei pesci su larga scala Queste sono aree che presentano condizioni ambientali idonee e risorse trofiche abbondanti che consentono ai giovanili di accrescersi velocemente. Questi ultimi, con l’aumentare delle dimensioni tendono ad abbandonare le aree di nursery per riunirsi progressivamente con la popolazione di origine, compiendo una migrazione controcorrente. La maggior parte delle specie ittiche demersali, distribuite nelle aree temperate, si contraddistinguono infatti per la presenza di aree di riproduzione e di nursery , queste ultime poste generalmente a minore profondità. Sono anche ben conosciute le migrazioni , spesso su larga scala, effettuate da alcune specie pelagiche di Teleostei, come il tonno rosso (Thunnus thynnus).
Alcune specie, dette diadrome, compiono migrazioni riproduttive dal mare alle acque interne o viceversa. Si riconoscono quindi 2 tipi di migrazioni:
Migrazioni anadrome: se la specie migra dal mare verso le acque interne (es. salmoni, alosa);Migrazioni catadrome: se la specie migra dalle acque interne verso il mare (es. anguilla).LA MIGRAZIONE DEL TONNO Il tonno rosso è un animale gregario che forma branchi numerosissimi composti da individui di dimensioni simili che quindi condividono le stesse esigenze alimentari e la stessa velocità di nuoto.
Il tonno rosso (Thunnus thynnus ), è presente in Atlantico con due sotto-polazioni, una composta da individui che migrano lungo le coste orientali del sud e nord America e che si riproducono nel Golfo del Messico e l’altra distribuita lungo le coste occidentali atlantiche, dal Marocco alla Norvegia, che si riproduce in Mediterraneo. Le due popolazioni non sono separate ma viceversa sono frequenti le migrazioni attraverso l’Atlantico. La migrazione dei tonni da una sponda all’altra dell’Atlantico dura circa 4 mesi per compiere 9.000 Km. La media è quindi di 40 miglia al giorno . Il tonno rosso è un animale gregario che forma branchi numerosissimi composti da individui di dimensioni simili che quindi condividono le stesse esigenze alimentari e la stessa velocità di nuoto. La formazione di banchi riguarda soprattutto i giovanili e gli adulti in fase riproduttiva. La riproduzione in Mediterraneo avviene a Maggio-Giugno , quando i grandi tonni di peso superiore a 100 Kg e 10 anni di età, detti “tonni di corsa” entrano da Gibilterra e seguono le diramazioni della corrente di ingresso atlantica per raggiungere le aree di riproduzione, situate nella parte occidentale del bacino, soprattutto tra Sardegna, Sicilia ma anche nel mar Ionio e nell’Adriatico centro-meridionale.
Al momento della deposizione dei gameti i pesci iniziano a nuotare vorticosamente e formano un enorme cilindro rotante da cui i pesci scattano a turno verso il centro.
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